The purpose of this review has been to draw the attention of clinicians towards the possibility that some of the patients they are treating for thrombosis may have an underlying immune disturbance. This could involve functional abnormalities of the complement system (as in acquired angioneurotic oedema or in paroxysmal nocturnal haemoglobinuria), or cell-mediated immunological damage to the vessel wall (as in Behcet's syndrome or Buerger's disease), or the presence of circulating antibodies (the lupus anticoagulant or antibodies to heparin). While obviously our knowledge on most aspects is still very incomplete, the awareness of the association of thrombosis with certain immune disorders should encourage further detailed studies of mechanisms and enhance our understanding of the role of blood constituents and the vessel wall in thrombogenesis.